Semiconductor memory devices may be generally classified as either volatile or nonvolatile memory devices depending on whether the device is capable of retaining data when power to the device power is interrupted.
As the use of mobile devices, such as mobile phones, personal digital assistants (PDA), digital cameras, portable game consoles, MP3 players and the like increases, the use of nonvolatile memory devices as both data storage devices and as code storage devices has also increased. Nonvolatile memory devices are also being used with increased frequency in high definition televisions (HDTV), digital versatile disc (DVD) players, routers, global positioning systems (GPS) and various other applications.
Flash memory devices are one widely used type of nonvolatile memory device. Flash memory devices may be classified as either NOR flash memory devices or NAND flash memory devices based on the connection state of a cell transistor and a bit line. In a NOR flash memory device, two or more cell transistors are connected to one bit line in parallel. In a NAND flash memory device, two or more cell transistors are connected to one bit line in series. A NOR flash memory device can be read by accessing stored data in a byte-by-byte manner, while a NAND flash memory device has a high cell density. As such, NOR flash memory devices are particularly suitable as code storage devices, while NAND flash memory devices are particularly suitable as data storage devices.
The flash memory device in the past was limited to storing only one bit of data per memory cell. Recent technological advances, however, have allowed researchers to develop flash memory devices that store more than one bit of data per memory cell. A flash memory device that stores one bit of data per memory cell is referred to as a single level cell (SLC) device, and a flash memory device that stores more than one bit of data per memory cell is referred to as a multi-level cell (MLC) device. As the demand for small, high density memory devices has increased, MLC devices have been increasingly used in a wide variety of applications. However, since the MLC device stores data based on a plurality of threshold voltages, distributions of the threshold voltages may negatively affect reliability of the MLC device.